This invention relates to power plants utilizing at least one steam turbine and one gas turbine in combination with a heat recovery steam generator (HRSG) and in particular, relates to a means for controlling the temperature of the steam output of the HRSG into the steam turbine independently of the gas turbine operation.
Steam turbine start-up philosophy calls for minimizing temperature differences between steam temperature and steam turbine metal temperature to reduce the potential for steam turbine thermal cyclic damage. Therefore, combined cycle plant start-up modes are dictated by steam turbine metal temperature. When the steam turbine metal temperature is close to the maximum steam temperature able to be generated, no restrictions are imposed upon the gas turbine start-up and loading. When the steam turbine metal temperature is moderately or much colder than the maximum steam temperature able to be generated, then restrictions are imposed upon the gas turbine loading. When gas turbine loading is restricted, a lower gas turbine exhaust gas temperature results with a correspondingly lower steam temperature and steam flow being generated by the HRSG. Past combined cycle steam turbine sizes have not required, for start-up and initial loading, steam flows in excess of that produced by one gas turbine HRSG package; however, with larger steam turbines being used it now appears that if the current practice of restricting the load on the gas turbine is maintained, then two, rather than one, gas turbine HRSG packages will be required to generate ample steam flow to roll-off and initially load the larger steam turbines.
An alternate solution would be to employ a start-up attemperator in the main steam line adjacent to the HRSG steam header. The use of a steam desuperheater would obviate the restrictions on gas turbine loading along with associated systems complications. Steam flow generation could then be brought to maximum temperature and flow for the gas turbine/HRSG package thereby providing sufficient steam flow for steam turbine roll-off.
The addition of a start-up attemperator could be used to allow optimum design of the HRSG superheater to take place within the bounds of economic evaluation factors with the excess steam temperature reduced or shaved by the attemperator to values consistent with acceptable steam turbine operation. Plant usage as a temperature shaving device would be very remote requiring an ambient temperature extreme and gas turbine operation at peak. This condition conceivably might never happen in the life of the plant, but, however the plant must be designed for that eventuality.